1. Field of the Invention
This invention generally relates to a color signal processing circuit (hereunder sometimes referred to as a color signal circuit). More particularly, this invention relates to a color signal circuit, which can be applied to various equipment, having an automatic phase control (APC) circuit provided in a portion for generating a carrier to be used in a frequency conversion and color demodulation operations of a carrier chrominance signal, to a color signal circuit having an automatic color control function and to a hue control circuit for controlling a hue of a color indicated by a color difference signal.
2. Description of the Related Art
There has been a color signal circuit for use in various equipment such as a color television receiver and a color video tape recorder (VTR) which is provided with an APC circuit in a portion for generating a carrier to be used in the signal processing thereof.
FIG. 10 is a schematic block diagram for showing the construction of an example of a conventional APC circuit provided in the playback system of a color VTR which is adapted to shift the phase of a low frequency band carrier chrominance signal by 90 degrees every horizontal scanning period (hereunder sometimes referred to simply as 1-H period) and reverse the direction of the phase shift of the low frequency band carrier chrominance signal at every sequential video track and record and playback a signal obtained by the frequency division multiplexing of the low frequency band carrier chrominance signal and a low frequency band modulated luminance signal.
In the conventional APC circuit of FIG. 10, reference numeral 1 denotes an input terminal for receiving a reproduced color signal (that is, a reproduced low frequency band carrier chrominance signal); 2 an automatic chroma control circuit (hereunder referred to as a color control (ACC) circuit) for automatically adjusting the signal level of a color signal to a predetermined value; 3 and 9 frequency conversion circuits; 4 and 10 band-pass filters (hereunder sometimes abbreviated as BPFs); 5 (1/2)-burst circuit; 6 a 1-H delay line; 7 a color killer circuit; 8 an automatic frequency control (AFC) circuit; 11 a variable crystal oscillator (VCO); 12 a reference crystal oscillator; 13 and 16 phase comparison circuits; 14 a burst gate circuit; 15 a color killer detection circuit; 17 a phase shifting circuit (hereunder sometimes referred to as a phase shifter) for shifting the phase of an input signal thereto by 90 degrees; and 18 an adder. Incidentally, a reference chrominance subcarrier is supplied from the crystal oscillating circuit 12 to the phase comparison circuit 13. Further, the reproduced low frequency band carrier chrominance signal is supplied from the AFC circuit to the frequency conversion circuit.
In case of this APC circuit, first, the ACC circuit 2 makes the signal level of a low frequency band carrier chrominance signal constant and then supplies the low frequency band carrier chrominance signal to the frequency conversion circuit 3. Subsequently, a carrier chrominance signal is obtained by effecting a frequency conversion operation in the frequency conversion circuit 3. Further, a well-known automatic phase control operation is performed on the thus obtained carrier chrominance signal by a closed loop comprised of the frequency conversion circuit 3, the band-pass filter 4, the (1/2)-burst circuit 5, a comb type filter, which is composed of the 1-H delay line 6 and the adder 18, the burst gate circuit 14, the phase comparison circuit 13, the VCO 11, the frequency conversion circuit 9 and the band-pass filter 10.
Furthermore, in this conventional APC circuit, the phase of a color burst signal obtained by the burst gate circuit 14 and that of the reference chrominance subcarrier issued by the crystal oscillating circuit 12 are compared by the phase comparison circuit 13 which outputs a phase error signal to the VCO 11 to change the frequency of the oscillation thereof.
If the conventional APC circuit has infinite d.c. gain and does not include a delaying element such as a loop filter, a phase control operation of controlling the change of the phase of the reproduced color signal (that is, the reproduced low frequency band carrier chrominance signal) can be performed without delay time. The APC circuit, however, cannot have infinite d.c. gain. In other words, a practical APC circuit has inevitably a residual phase error. Further, in case where a phased lock loop (PLL) type circuit is employed as the phase comparison circuit, a residual phase error is caused due to an operating principle of the PLL. Thus, the APC circuit effects an APC operation at a finite speed of response.
Further, the reproduced color signal (that is, the reproduced low frequency band carrier chrominance signal) are usually mixed with noises resulting from various origin, so that the signal-to-noise ratio (S/N) is deteriorated.
Furthermore, in case where the color burst signal obtained by the burst gate circuit 14 of the conventional APC circuit is mixed with noises, the phase comparison circuit 13 outputs a phase error signal caused due to the presence of noises even when the phase of the color burst signal obtained by the burst gate circuit 14 is correct.
The APC circuit performs the APC operation in such a manner to cancel the phase error signal having occurred in response to the noises which are mixed with the color burst signal. Thus, in case where the reproduced color signal is mixed with noises, the phase of the reproduced color signal fluctuate as a result of the operation of the APC circuit. Further, this results in the occurrence of color deviation in a reproduced image.
The color deviation thus generated in the reproduced image, however, can be reduced to some extent by reducing the speed of response of the APC circuit. Therefore, the APC operation has been performed by the conventional APC circuit under the conditions that an appropriate value of the residual phase error is present.
However, in case of the APC circuit adapted to perform the APC operation in spite of the presence of the residual phase error, a stripe-shaped color deviating portion is generated at specific locations in an image reproduced from the reproduced color signal sent from a color VTR in which, for example, what is called a head beat phenomenon may be caused.
As is well known in the art, in case where a head drum, which has two video heads at the positions of central symmetry on opposite sides thereof, has an additional head at a specific location between the two video heads, this phenomenon may happen when the relative speed of a magnetic tape to the video heads is changed by vibration caused in the tape at the times of making the tape, which is in contact with the video heads, touch the additional head and releasing the tape from the additional head and further the APC circuit cannot respond to change in frequency of the reproduced color signal caused by the change of the relative speed of the tape.
Furthermore, the stripe-shaped color deviating portion generated due to such a phenomenon in the reproduced image is inconspicuous in case that the S/N is deteriorated. In contrast, in case that the S/N is ameliorated by employing, for instance, a recursive type noise reducing circuit including a frame memory or field memory, the color deviating portion becomes clearly perceived.
In order to prevent the occurrence of the color deviation due to such a phenomenon, it is necessary to increase the speed of response of the APC circuit as far as possible. However, as stated above, when the speed of response of the APC circuit is increased, the problem of the color deviation generated in the reproduced image due to the noises mixed up with the color burst signal is brought up. Further, as above described, the conventional APC is adapted to perform the APC operation under the conditions that an appropriate value of the residual phase error is present. Therefore, the conventional APC circuit cannot eliminate or reduce the chance of the occurrence of the color deviation due to the head beat phenomenon.
The present invention is accomplished to resolve the above described problem (hereunder sometimes referred to as a first problem) of the conventional color signal circuit.
It is accordingly an object (hereunder sometimes referred to as a first object) of the present invention to provide a color signal circuit which can easily obtain a color difference signal from which the residual phase error made by the APC operation of the APC circuit to be included in a color signal.
Further, when the APC loop is unlocked due to some cause in the APC circuit of which the response is delayed as described above, there is caused the color deviation in the reproduced image until the APC circuit is restored to a normal APC operating condition.
The color deviation originated from the unlocking of the APC loop of the ACC circuit is often caused due to, for example, the dropout of the reproduced color signal generated by repeating a duplicating operation in the VTR adapted to performing the processing of the low frequency band carrier chrominance signal and the deterioration of the S/N. Especially, a color flicker can be easily caused in an upper portion of the reproduced image by the unlocking of the APC loop occurring immediately posterior to the vertical synchronization signal due to the fact that a color burst signal is not present in a vertical synchronization signal period.
By increasing the speed of response of the APC circuit as far as possible, this problem can be resolved. However, as above described, the conventional APC is adapted to perform the APC operation under the conditions that an appropriate value of the residual phase error is present. Thus, the conventional APC circuit cannot eliminate or reduce the chance of the occurrence of the color flicker.
The present invention is accomplished to also resolve the above described problems of the conventional color signal circuit.
Further, in various equipment such as a color television receiver and a color video tape recorder (VTR), has been used a conventional ACC circuit, for example, as shown in FIG. 29.
In the conventional ACC circuit of FIG. 29, reference numeral 124 denotes a video head; 125 a playback amplifier; 126 a variable gain control circuit; 127 a frequency conversion circuit; 128 a comb type filter; 129 a burst gate circuit; 130 an amplitude detection circuit; and 131 a mixer. Further, a signal reproduced by the video head 124 is amplified by the playback amplifier 125 and is thereafter supplied to the variable gain control circuit 126.
Furthermore, this conventional ACC circuit, which includes a closed loop automatic control system comprised of the variable gain control circuit 126, the frequency conversion circuit 127, the comb type filter 128, the burst gate circuit 129 and the amplitude detection circuit 130, performs an automatic control operation in such a manner to make the amplitude of the color burst signal obtained by the burst gate circuit 129 constant.
This conventional ACC circuit, however, has drawbacks in that the range, in which the output signal level is constant, is narrow owing to the limit to the operating range by the operating source voltage thereof and to the finiteness of the d.c. gain thereof and that the signal level of the output signal is fluctuated when the color burst signal input thereto is mixed with a noise.
Thus, the conventional ACC circuit is made to have a relatively large time constant of the loop filter in order to reduce the fluctuation of the signal level of the output signal caused due to the noise.
Further, in case of the VTR of which a plurality of video heads are sequentially switched from each other with a lapse of time, the color flicker is generated in the reproduced image and thus the picture quality of the reproduced image is deteriorated if there is any difference among the signal levels of the signals reproduced by the plurality of the video heads.
Even in case where there is any difference among the signal levels, if the difference can be compensated by the operation of the ACC circuit, such difference is no consideration. However, as illustrated in FIG. 30, the input/output characteristic of the conventional ACC circuit is not sufficient to compensate the difference among the signal levels of the reproduced color signals. Thus, in case of employing the conventional ACC circuit, the color flicker often occurs.
Further, even in case where the color flicker is generated in the reproduced image due to the above described cause, if the S/N of the color signal is deteriorated, the color flicker may be masked by the noise, so that viewers may not mind the color flicker. In contrast, if the S/N of the reproduced color signal is ameliorated by using, for instance, the recursive type noise reducing circuit which includes the frame memory or the field memory, the color flicker becomes clearly perceived in the reproduced image by the viewers.
The present invention is accomplished to also resolve this problem of the conventional color signal circuit having the ACC circuit.
It is accordingly another object (hereunder sometimes referred to as a second object) of the present invention to provide a color signal circuit which can eliminate or reduce the chance of the occurrence of the color flicker.
Further, in various equipment such as a color television receiver and a color VTR, the conventional process of controlling hue is performed by changing the phase of the carrier used for obtaining the carrier chrominance signal or changing the phase of the color burst signal.
The conventional phase shifter for shifting the phase of the color burst signal and that of the carrier used for obtaining the carrier chrominance signal is usually comprised of an analog circuit. Thus, the characteristics of the conventional phase shifter are liable to largely vary in response to change in ambient temperature. In the prior art, it is therefore very difficult to obtain a hue control device which can stably control the hue.
Furthermore, in the prior art, there is no means for easily controlling the hue of the color indicated by the color signal which is in the form of the color difference signal. Thus, in the conventional equipment which processes the color difference signal, the hue cannot be easily controlled.
The present invention is accomplished to also eliminate this drawback of the prior art.
It is therefore still another object (hereunder sometimes referred to as a third object) of the present invention to provide a color signal circuit which can easily control the hue.